Rigid polyethylene reinforced composites having improved short beam shear strength

ABSTRACT

The present invention relates to high strength, high modulus polyethylene filaments which form composites having improved shear strength and composites formed from said filaments.

This application is a continuation of application Ser. No. 594,579,filed Oct. 9,1990 which is a division of application Ser. No. 367,642,filed. Jun. 19, 1989, now U.S. Pat. No. 5,006,390.

BACKGROUND OF THE INVENTION

The invention relates to rigid composite articles reinforced with hightenacity, high modulus polyethylene fibers. More particularly, thisinvention relates to such articles having improved short beam shearstrength.

PRIOR ART

Fiber reinforced composites are well known in the prior art. Suchcomposites are composed of a fiber or reinforcing phase and a matrixphase. Many fiber and matrix material are available and a great numberand variety of composite compositions may be and have been prepared.Among the fibers available for composite construction are: glass,carbon, aramid and more recently, high tenacity, high moduluspolyethylene. For example, polyethylene fibers and yarns of hightenacity exceeding 20 g/d and modulus exceeding 500 g/d have beendescribed for example in U.S. Pat. No. 4,137,394 to Methuzen et al.,U.S. Pat. No. 4,356,138 of Kavesh et al., and U.S. Pat. No. 4,344,908 toSmith et al.

The matrix material employed may vary widely and may be a metallicmaterial, a semi-metallic material, an organic material and/or aninorganic material. The matrix material may be flexible (low modulus) orrigid (high modulus). Among the useful high modulus or rigid matrixmaterials are thermoplastic resins such as polycarbonates,polyphenylenesulfides, polyphenylene oxides, polyester carbonates,polyesterimides, and polyimides; and thermosetting resins such as epoxyresins, phenolic resins, modified phenolic resins, allylic resins, alkydresins, unsaturated polyesters, aromatic vinylesters as for example thecondensation produced of bisphenol A and methacrylic acid diluted in avinyl aromatic monomer (e.g. styrene or vinyl toluene), urethane resinsand amino (melamine and urea) resins. The major criterion is that thematrix material holds the filaments together, and maintains thegeometrical integrity of the composite under the desired use conditions.

In particular, composites containing high strength, high moduluspolyethylene fibers in combination with rigid matrices such as epoxyresins have been described in U.S. Pat. Nos. 4,403,012, 4,455,273,4,543,286 and 4,623,574 to Harpell et al., and in an article entitled"Properties of a Polymer-matrix Composite Incorporating ALL160 A-900Polyethylene Fiber" by D,F. Adams et al., Sampe J., September/October,pp. 44-48, (1985). However, all such known composites containing highstrength, high modulus polyethylene fibers in combination with rigidmatrices have exhibited low shear properties. For example, in theaforementioned article by D,F, Adams et al., the authors state that " .. . shear properties are somewhat low". The significance of thisobservation is that for particular applications, e.g., aircraftstructural composites, shear strength specifications are vitallyimportant to the utility of the composite.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the invention and the accompanying drawings in which:

FIG. 1 is a plot of experimentally obtained short beam shear strengthvalues versus calculated short beam shear strength values.

FIG. 2 is a plot of experimentally obtained short beam shear strengthvalues vs. short beam shear strength values calculated from theregression analysis.

SUMMARY OF THE INVENTION

The present invention relates to high strength, high moduluspolyethylene filaments which form composites having improved shearstrength, said filaments having a tenacity of at least about 20grams/denier, a tensile modulus of at least about 500 grams/denier, anenergy-to-break of at least about 30 joules/gram, a weight averagefilament diameter equal to or less than about 30 micrometers and anaverage filament aspect ratio equal to or less than about 2.9:1. As usedherein, the "weight average filament diameter", "D" is determined asfollows:

    D=12.01Denier/Fil,micrometers

where "denier" is the total yarn denier and "Fil" is the number offilaments in the yarn.

To determine the average filament aspect ratio, a scanning electronmicrograph is taken of the cross-section of the yarn. The picture ismounted on the stage of an epidiascope of a Leitz T.A.S. image analyzer.The image analyzer determines for each filament the lengths of thelongest and shortest chords through the centroid of the filamentcross-section. The aspect ratio for a particular filament is the ratioof these dimensions. The average filament aspect ratio for the yarn isthe arithmetic average of the aspect ratios of the individual filaments.

Another aspect of this invention relates to composites fabricated fromthe fibers of this invention. These composites comprise a network of thehigh strength polyethylene fibers of this invention in a matrixmaterial.

Surprisingly, the composites of this invention exhibit improved shortbeam shear strength compared to the short beam shear strength ofcomposites incorporating filaments of larger weight average diameters orlarger average filament aspect ratio where all other physicalcharacteristics such as fiber volume fractions, fiber, matrix materialand the like are the same. As used herein the "short beam shearstrength" of a composite is the apparent interlaminar shear strength ofa parallel fiber reinforced composite and is determined by the shortbeam method as described in A.S.T.M. Standard Test Method D 2344-76.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of this invention relates to high strength, high moduluspolyethylene filaments having a tenacity of at least about 20grams/denier, a tensile modulus of at least about 500 grams/denier, anenergy-to-break of at least about 30 joules/gram, said filaments havinga weight average filament diameter equal to or less than about 30micrometers, and an average filament aspect ratio equal to or less thanabout 2.9:1. As used herein, the term "polyethylene" shall mean apredominantly linear polyethylene material that may contain minoramounts of chain branching or comonomers not exceeding 5 modifying unitsper 100 main chain carbon atoms, and that may also contain admixedtherewith not more than about 50 wt % of one or more polymeric additivessuch as alkene-1-polymers, in particular low density polyethylene,polypropylene or polybutylene, copolymers containing mono-olefins asprimary monomers, oxidized polyolefins, graft polyolefin copolymers andpolyoxymethylenes, or low molecular weight additives such asanti-oxidants, lubricants, ultra-violet screening agents, colorants andthe like which are commonly incorporated by reference. Depending uponthe formation technique, the draw ratio and temperatures, and otherconditions, a variety of properties can be imparted to these filaments.Many of the filaments have melting points higher than the melting pointof the polymer from which they were formed. Thus, for example, highmolecular weight polyethylenes of 150,000, one million and two milliongenerally have melting points in the bulk of 138° C. The highly orientedpolyethylene filaments made of these materials have melting points offrom about 7 to about 13° C. higher. Thus, a slight increase in meltingpoint reflects the crystalline perfection and higher crystallineorientation of the filaments as compared to the bulk polymer.Surprisingly, we have discovered that the weight average diameter andthe average aspect ratio of polyethylene filaments has an effect on theshort beam shear strength of composites formed from the filaments. Inthe preferred embodiments of the invention, the weight average diameterof the filaments is from about 20 to about 30 micrometers, and theaverage aspect ratio of the filaments is from about 2.5:1 to about2.9:1. In the particularly preferred embodiments of the invention, theweight average diameter of the filaments is from about 15 to about 20micrometers, and the average aspect ratio of the filaments is from about2:1 to about 2.5:1. In the most preferred embodiments of the inventionthe weight average diameter of the filaments is from about 5 to about 15micrometers, and the average aspect ratio of the filaments is from about1:1 to about 2:1.

Preferred polyethylene filaments for use in the practice of thisinvention are those having a tenacity equal to or greater than about 20g/d, a tensile modulus equal to or greater than about 500 g/d, anenergy-to-break equal to or greater than about 20 joules/grams and amolecular weight equal to or greater than about 150,000. Particularlypreferred filaments are those having a tenacity equal to or greater thanabout 25 g/d, a tensile modulus equal to or greater than about 1000 g/d,energy-to-break equal to or greater than about 30 joules/grams and amolecular weight equal to or greater than about 500,000. Amongst theseparticularly preferred embodiments, most preferred are those embodimentsin which the tenacity of the filaments are equal to or greater thanabout 30 g/d, the tensile modulus is equal to or greater than about 1200g/d, the energy-to-break is equal to or greater than about 40joules/gram and a molecular weight equal to or greater than about1,000,000. In the practice of this invention, filaments of choice have atenacity equal to or greater than about 30 g/d, the tensile modulus isequal to or greater than about 1200 g/d, the energy-to-break is equal toor greater than about 40 joules/gram and a molecular weight of fromabout 1,000,000 to about 5,000,000.

High strength, high modulus polyethylene filaments for use in thepractice of this invention may be formed using conventional procedures.For example, such filaments may be grown in solution as described inU.S. Pat. No. 4,137,394 to Meihuzen et al, or U.S. Pat. No. 4,356,138 ofKavesh et al., issued Oct. 26, 1982, or a filament spun from a solutionto form a gel structure, as described in German Off. 3,004,699 and GB2051667, and especially as described in application Ser. No. 572,607 ofKavesh et al. filed Jan. 20, 1984 (see EPA 64,167 published Nov. 10,1982).

Another aspect of this invention relates to a composite comprising anetwork of the polyethylene filaments of this invention in a matrix. Inthe composite articles of our invention, the filaments may be arrangedin networks having various configurations. For example, a plurality offilaments can be grouped together to form twisted or untwisted yarnbundles in various alignment. In preferred embodiments of the invention,the filaments in each layer are aligned substantially parallel andunidirectionally such that the matrix material substantially coats theindividual filaments. The filaments or yarn may be formed as a felt,knitted or woven (plain, basket, sating and crow feet weaves, etc.) intoa network, fabricated into non-woven fabric, arranged in parallel array,layered, or formed into a fabric by any of a variety of conventionaltechniques. Among these techniques, for ballistic resistanceapplications we prefer to use those variations commonly employed in thepreparation of aramid fabrics for ballistic-resistant articles. Forexample, the techniques described in U.S. Pat. No. 4,181,768 and in M.R. Silyquist et al., J. Macromol Sci. Chem., A7(1), pp. 203 et. seq.(1973) are particularly suitable.

The matrix material employed in the conduct of this invention may varywidely and may be a metallic material, a semi-metallic material, anorganic material and/or an inorganic material that having a modulusequal to or greater than about 50,000 psi (0.39 mPa). Illustrative ofuseful matrix materials are thermoplastic resins such as polycarbonates,polyarylene sulfides, polyarylene oxides, polyester carbonates,polyesterimides and polyimides; and thermosetting resins such as epoxyresins, phenolic resins, modified phenolic resins, allylic resins, alkydresins, unsaturated polyester, aromatic vinylester, urethane resins andamino resins.

In the preferred embodiments of the invention the matrix material is arigid (high modulus) polymeric material having a tensile modulusmeasured at about 23° C. equal to or greater than about 50,000 psi(344.7 MPa). Preferably the tensile modulus of the rigid matrix materialis greater than 100,000 psi (689.4 MPa), more preferably is greater than200,000 psi (1378.8 MPa) and most preferably is greater than about300,000 psi (2068.2 MPa).

The composite of this invention can be fabricated using a number ofprocedures. In general, the layers are formed by molding the combinationof the matrix material and filaments in the desired configurations andamounts by subjecting them to heat and pressure.

The filaments may be premolded by subjecting them to heat and pressure.For polyethylene filaments, molding temperatures range from about 20 toabout 150° C., preferably from about 80 to about 145° C., morepreferably from about 100 to about 135° C., and more preferably fromabout 110 to about 130° C. The pressure may range from about 10 psi (69kPa) to about 10,000 psi (69,000 kPa). A pressure between about 10 psi(69 kPa) and about 100 psi (690 kPa), when combined with temperaturesbelow about 100° C. for a period of time less than about 1.0 min, may beused simply to cause adjacent filaments to stick together. Pressuresfrom about 100 psi to about 10,000 psi (69,000 kPa), when coupled withtemperatures in the range of about 100 to about 155° C. for a time ofbetween about 1 to about 5 min, may cause the filaments to deform and tocompress together (generally in a film-like shape). Pressures from about100 psi (690 kPa) to about 10,000 psi (69,000 kPa), when coupled withtemperatures in the range of about 150 to about 155° C. for a time ofbetween 1 to about 5 min, may cause the film to become translucent ortransparent.

The filaments and networks produced therefrom are formed into "simplecomposites" as the precursor to preparing the complex composite articlesof the present invention. The term, "simple composites", as used hereinis intended to mean composites made up of one or more layers, each ofthe layers containing filaments as described above with a single majormatrix material, which material may include minor proportions of othermaterials such as fillers, lubricants, or the like as noted heretofore.

The proportion of rigid matrix material to filament is variable for thesimple composites, with matrix material amounts of from about 5% toabout 150 Vol. %, by volume of the filament, representing the broadgeneral range. Within this range, it is preferred to use compositeshaving a relatively high filament content such as composites having onlyabout 10 to about 50 Vol. % matrix material, by volume of the composite,and more preferably from about 10 to about 30 Vol. % matrix material byvolume of the composite.

Stated another way, the filament network occupies different proportionsof the total volume of the simple composite. Preferably, however, thefilament network comprises at least about 30 volume percent of thesimple composite. For ballistic protection, the filament networkcomprises at least about 50 volume percent, more preferably about 70volume percent, and most preferably at least about 75 volume percent,with the matrix occupying the remaining volume.

A particularly effective technique for preparing a preferred compositeof this invention comprised of substantially parallel, unidirectionallyaligned filaments includes the steps of pulling a filament or bundles offilaments through a bath containing a solution of a matrix material, andcircumferentially winding this filament into a single sheet-like layeraround and along a bundle of filaments the length of a suitable form,such as a cylinder. The solvent is then evaporated leaving a sheet-likelayer of filaments embedded in a matrix that can be removed from thecylindrical form. Alternatively, a plurality of filaments or bundles offilaments can be simultaneously pulled through the bath containing asolution or dispersion of matrix material and laid down in closelypositioned, substantially parallel relation to one another on a suitablesurface. Evaporation of the solvent leaves a sheet-like layer comprisedof filaments which are coated with the matrix material and which aresubstantially parallel and aligned along a common filament direction.The sheet is suitable for subsequent processing such as laminating toanother sheet to form composites containing more than one layer.

Similarly, a yarn-type simple composite can be produced by pulling agroup of filament bundles through a dispersion or solution of the matrixmaterial to substantially coat each of the individual filaments, andthen evaporating the solvent to form the coated yarn. The yarn can then,for example, be employed to form fabrics, which in turn, can be used toform more complex composite structures. Moreover, the coated yarn canalso be processed into a simple composite by employing conventionalfilament winding techniques; for example, the simple composite can havecoated yarn formed into overlapping filament layers.

The number of layers included in the composite of this invention mayvary widely depending on the uses of the composite, for example, inthose uses where the composite would be used as ballistic protection,the number of layers would depend on a number of factors including thedegree of ballistic protection desired and other factors known to thoseof skill in the ballistic protection art. In general for thisapplication, the greater the degree of protection desired the greaterthe number of layers included in the article for a given weight of thearticle. Conversely, the lessor the degree of ballistic protectionrequired, the lesser the number of layers required for a given weight ofthe article. It is convenient to characterize the geometries of suchcomposites by the geometries of the filaments and then to indicate thatthe matrix material may occupy part or all of the void space left by thenetwork of filaments. One such suitable arrangement is a plurality oflayers or laminates in which the coated filaments are arranged in asheet-like array and aligned parallel to one another along a commonfilament direction. Successive layers of such coated unidirectionalfilaments can be rotated with respect to the previous layer. An exampleof such laminate structures are composites with the second, third,fourth and fifth layers rotated +45°, -45°, 90° and 0°, with respect tothe first layer, but not necessarily in that order. Other examplesinclude composites with 0°/90° layout of yarn or filaments.

One technique for forming composites of this invention having more thanone layer includes the steps of arranging coated filaments into adesired network structure, and then consolidating and heat setting theoverall structure to cause the coating material to flow and occupy theremaining void spaces, thus producing a continuous matrice. Anothertechnique is to arrange layers or other structures of coated or uncoatedfilament adjacent to and between various forms, e.g. films, of thematrix material and then to consolidate and heat set the overallstructure. In the above cases, it is possible that the matrix can becaused to stick or flow without completely melting. In general, if thematrix material is caused to melt, relatively little pressure isrequired to form the composite; while if the matrix material is onlyheated to a sticking point, generally more pressure is required. Also,the pressure and time to set the composite and to achieve optimalproperties will generally depend on the nature of the matrix material(chemical composition as well as molecular weight) and processingtemperature.

The composites of this invention comprising one or more layers may beincorporated into complex composites. For example, such composites maybe incorporated into more complex composites to provide a rigid complexcomposite article suitable, for example, as structuralballistic-resistant components, such as helmets, structural members ofaircraft, and vehicle panels.

For example, a particularly useful ballistic-resistant complex compositecomprises a simple composite comprising high strength filaments in a lowmodulus elastomeric matrix on which is formed at least one layercomprising highly-orientated ultra high molecular weight polyethylenefilament in a rigid matrix, such as an epoxy resin. Other useful complexballistic resistant composites comprises one or more layers comprisinghighly-oriented ultra high molecular weight polyethylene filaments ofthis invention in a rigid matrix such as an epoxy resin and one or morelayers formed from an impact resistant material such as steel plates,titanium plates, composite armor plates, ceramic reinforced metalliccomposites, ceramic plates and high strength filament composites such asS-glass, E-glass or aramifilaments in a high modulus resin matrix. Amost preferred embodiment of the invention is a complex composite havingone or more layers formed from a network comprising the filaments ofthis invention in a rigid matrix, and one or more rigid impact resistantlayers which will at least partially deform the initial impact surfaceof the projectile or cause the projectile to shatter such as layersformed from a ceramic material as for example aluminum oxide, boroncarbide, silicon carbide and titanium borides.

The composites of this invention exhibit improved short beam shearstrength as compared to analogous composites formed from filaments oflarger diameter and/or larger aspect ratio. In the preferred embodimentsof the invention, the short beam shear strength of the composites areequal or greater than that given by the following relationship:

    SSBS(KSI)=7.32-10.73 V.sub.f +6.99 V.sub.f.sup.2 +K.sub.t

wherein:

SBSS is the short beam shear strength in KSI units;

V_(f) is the fiber volume fraction in the composite; and

K_(t) is a factor related to the surface treatment of the yarn whereK_(t) is equal to about 0 when the surface of the yarn is subjected tocorona discharge treatment, and K_(t) is equal to 0.47 when the surfaceof the yarn is subjected to plasma treatment.

The composites of this invention are useful in the fabrication of manyuseful articles such as ballistic armor and helmets, or structuralcomposites.

The following examples are presented to provide a more completeunderstanding of the invention. The specific techniques, condition,materials, proportions and reported data set forth to illustrate theprinciples of the invention are exemplary and should not be construed aslimiting the scope of the invention.

EXAMPLES 1 TO 15

(A) Yarn Preparation, Yarns A-F

High strength, high modulus polyethylene yarns were prepared from 22.7IV (3.2×10⁶ weight average molecular weight) polymer in accordance withthe procedures in U.S. Pat. No. 4,413,110 and Ser. No. 895,396 filedAug. 11, 1986. All yarns were of 118 filaments. Prior art yarns A & Bwere of weight average filament diameter larger than 30 μm and/oraverage filament aspect ratio greater than about 2.9:1. Yarns C-F ofthis invention were of weight average filament diameter less than 30 μmand of average filament aspect ratio less than 2.9:1.

The weight average filament diameters, the average filament aspectratios and the yarn tensile properties are set forth in the followingTable I.

                  TABLE I                                                         ______________________________________                                        POLYETHYLENE YARN AND FILAMENT PROPERTIES                                                  Wt. Avg.  Avg. Filament                                               Yarn    Filament  Aspect    Tenacity                                                                             Modulus                               Yarn Denier  Diam., μm                                                                            Ratio     g/d    g/d                                   ______________________________________                                        I. Prior Art Yarns                                                            A    1121    37.0      3.0       33     1420                                  B    1223    38.7      2.5       30     1231                                  II. Yarns of This Invention                                                   C    326     20.0      2.7       33     1609                                  D    295     19.0      2.4       34     1594                                  E    294     19.0      2.2       33     1684                                  F    295     19.0      2.2       32     1466                                  ______________________________________                                    

The yarns of this invention may be distinguished from prior art yarns bytheir combination of low filament diameter and high filamentcylindricity. The yarns of the invention are of weight average filamentdiameter less than about 30 μm and average filament aspect ratio lessthan about 2.9:1, while the weight average filament diameter of theprior art yarns was greater than about 35 μm and the average filamentaspect ratio of these yarns was greater than about 3.0:1.

Surface Treatment

Yarns A-F prepared for composite manufacture by corona or plasmatreatment. Corona treatment was carried out using a commercial coronatreatment unit model No. CMD-200-MM-PN-EX manufactured by Softal Co.This machine operates at a constant DC voltage of 5000 volts andvariable current. Treatment level also depends on treatment speed. Inthe following examples, all yarns which were corona treated were run at10ft/min (3.05 m/min) using 0.8 amp treatment current. Treatment levelwas 100 watts/ft² /min. (9.29 m² /min).

Plasma treatment was carried out using a system 8060 commercial unitmanufactured by Branson/IPC. The conditions under which the yarns wereplasma treated were:

Atmosphere: 0₂

Pressure: 0.5 torr (66.5 Pa)

Power: 75 watt

Yarn residence time: 30 sec

(C) Composite Manufacture and Testing

Short beam shear test specimens were prepared of 0.40 "-0.50" (1.02cm-1.27 cm) width, 1" (2.54 cm) length and 0.12 "-0.13" (0.30 cm-0.33cm) thickness. The matrix resin was Shell Epon 828, 30 PHR methylenedianaline (from Ciba-Geigy, designated HT-972), and 40 PHR Fortifier I(from Uniroyal Co.). Specimens were cured at 80° C. for 3 hours then at120° C. for 16 hours. The composites consisted of 10 to 90 vol % of theyarns embedded in a matrix material. Short beam testing was performed inaccordance with A.S.T.M. Standard Test Method D2344-26. The short beamstrength (SBSS) results and composite properties are presented in thefollowing Table II.

                  TABLE II                                                        ______________________________________                                        COMPOSITES PROPERTIES                                                                                      Fiber                                                             Surface     Volume SBSS                                      Example   Yarn   Treatment   Fraction                                                                             KSI                                       ______________________________________                                        I. Prior Art Yarns                                                             1        A      Corona      0.38   3.70                                       2        A      Corona      0.41   3.25                                       3        A      Corona      0.64   3.13                                       4        A      Corona      0.71   2.69                                       5        A      Plasma      0.45   4.20                                       6        A      Plasma      0.59   3.82                                       7        A      Plasma      0.66   3.10                                       8        A      Plasma      0.67   3.24                                       9        B      Corona      0.60   3.30                                      II. Yarns of This Invention                                                   10        C      Corona      0.49   4.5                                       11        D      Plasma      0.22   6.53                                      12        D      Plasma      0.54   4.50                                      13        E      Corona      0.27   6.01                                      14        F      Corona      0.42   4.90                                      15        F      Corona      0.35   5.00                                      ______________________________________                                    

The short beam strength of the composites of the invention are at leastas high as given by the following relationship:

    SSBS(KSI)=7.32 -10.73 V.sub.f +6.99 V.sub.f.sup.2 +K.sub.t (Eq.(2))

Where: SBSS is the short beam shear strength, Ksi;

V_(f) is the fiber volume fraction in the composite;

K_(t) is a factor related to the surface treatment of the yarn;

K_(t) =0 for corona treatment; and

K_(t) =0.47 for plasma treatment.

The experimental short beam shear strength (SBSS) and the short beamshear strength calculated from Eq.(2) for composites containing priorart yarns and for composites containing the yarns of the invention areset forth in the following Table III and Table IV respectively.

                  TABLE III                                                       ______________________________________                                        Composite Properties                                                          Prior Art Yarns                                                                                      Fiber          SBSS                                                 Surface   Volume   SBSS  Calc/From                               Ex  Yarn     Treatment Fraction KSI   Eq. 2                                   ______________________________________                                        1   A        Corona    0.38     3.70  4.25                                    2   A        Corona    0.41     3.25  4.10                                    3   A        Corona    0.64     3.13  3.32                                    4   A        Corona    0.71     2.69  3.22                                    5   A        Plasma    0.45     4.20  4.38                                    6   A        Plasma    0.59     3.82  3.89                                    7   A        Plasma    0.66     3.10  3.75                                    8   A        Plasma    0.67     3.24  3.74                                    ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        Composite Properties                                                          Yarns of the Invention                                                                               Fiber          SBSS                                                 Surface   Volume   SBSS  Calc/From                               Ex  Yarn     Treatment Fraction KSI   Eq. 2                                   ______________________________________                                        10  C        Corona    0.49     4.5   3.74                                    11  D        Plasma    0.22     6.53  5.77                                    12  D        Plasma    0.54     4.50  3.74                                    13  E        Corona    0.27     6.01  4.93                                    14  F        Corona    0.42     4.90  4.05                                    15  F        Corona    0.35     5.00  4.42                                    ______________________________________                                    

For the composites prepared from the prior art yarns, the experimentallymeasured SBSS was less than that calculated from Eq.(2). In contrast,for each of the composites of this invention prepared using the yarns ofthis invention, the measured SBSS was greater than that calculated fromEq.(2). This result is shown graphically in FIG. 1.

In FIG. 1, the experimental SBSS is plotted versus the SBSS calculatedfrom Eq.(2). For each of the composites of the invention theexperimental SBSS lies above the calculated line, whereas for the priorart composites the experimental SBSS lies below the calculated line.

(D) Data Analysis

In order to determine the relationships of SBSS to yarn and treatmentparameters, the data of Tables I and II were subjected to statisticalanalysis by multiple linear regression. The regression equation obtainedfor SBSS was as follows:

    SBSS(KSI)10.11-10.73V.sub.f +6.99 V.sub.f.sup.2 +K.sub.t -0.048D-0.480AR(Eq.(1))

Where: V_(f) is the fiber volume fraction in the composite;

K_(t) is a factor related to the surface treatment of the yarn;

K_(t) =0 for corona treatment;

K_(t) =0.47 for plasma treatment;

D is the weight average filament diameter in the yarn, micrometers; and

AR is the average filament aspect ratio.

The statistics of the regression were:

F ratio (5,14)-37.3

Significance level=99.9+%

Multiple correlation coefficient=0.9767

Standard error of the estimate=0.30Ksi

A comparison of the SBSS values set forth in Table II and thosecalculated from the regression are shown in FIG. 1. The regression showsthat increased short beam shear strength was achieved by decreased fibervolume fraction; use of plasma rather than corona treatment of the yarn;use of yarn with smaller weight average filament diameter; and/or use ofyarn with lower average filament aspect ratio.

What is claimed is:
 1. Polyethylene filaments having a tensile modulusof at least about 500 g/denier, an energy-to-break of at least about 20j/g, a tenacity equal to or greater than about 20 g/denier, a weightaverage filament diameter equal to or less than about 30 micrometers, anaverage filament aspect ratio equal to or less than 2.9:1 and amolecular weight of at least about 150,000.
 2. A filament as recited inclaim 1 wherein said filament diameter is from about 20 to about 30micrometers.
 3. A filament as recited in claim 2 wherein said filamentdiameter is from about 15 to about 20 micrometers.
 4. A filament asrecited in claim 3 wherein said filament diameter is from about 5 toabout 15 micrometers.
 5. A filament as recited in claim 4 wherein saidfilament diameter is from about 5 to about 30 micrometers.
 6. A filamentas recited in claim 1 wherein said filaments have a tenacity equal to orgreater than about 25 g/d, a tensile modulus equal to or greater thanabout 1000 g/d and an energy-to-break equal to or greater than about 30j/g.
 7. A filament as recited in claim 1 wherein said tenacity is equalto or greater than about 30 g/d, said modulus is equal to or greaterthan about 1300 g/d and said energy-to-break is equal to or greater thanabout 40 j/g.
 8. A filament as recited in claim 1 wherein said filamentaspect ratio is from about 2.5:1 to about 2.9:1.
 9. A filament asrecited in claim 8 wherein said aspect ratio is from about 2:1 to about2.5:1.
 10. A filament as recited in claim 9 wherein said aspect ratio isfrom about 1:1 to about 2:1.
 11. A filament as recited in claim 10wherein said aspect ratio is from about 1:1 to about 2.9:1.
 12. Afilament as recited in claim 1 wherein said molecular weight is equal toor greater than about 500,000.
 13. A filament as recited in claim 12wherein said molecular weight is equal to or greater than about1,000,000.
 14. A yarn as recited in claim 13 wherein said molecularweight is from about 1,000,000 to about 5,000,000.